1. Field of the Invention
This invention relates generally to a lubricating film thickness measuring system and method and more particularly to capacitive measurement of elastohydrodynamic film thickness in a rolling element bearing.
2. Description of the Prior Art
In lubrication systems particularly for gyroscopic applications, it is often desirable to maintain minimum drag torque and consistent performance over long periods of time. Increases in life requirements for lubricated parts have motivated the search for a lubrication system that would provide a constant amount of lubricating medium over a long period of time. In bearings, it is critical to provide a substantially exact amount of oil between the frictional surfaces since excessive oil has been shown to cause high drag torques due to excessive viscous drag, reduced stability of the rolling element retainer (the cage) and increased emitted vibration. Insufficient oil produces a thinner elastohydrodynamic film which leads to higher friction, increased wear and, consequently, premature bearing failure.
In a system of providing oil to a running, rolling element bearing through the use of an active control system which monitors oil requirements and injects oil when necessary, known as an active, autonomous oil lubrication system, it is necessary to know what the operating elastohydrodynamic (EHD) film thickness is in a ball bearing under various conditions. EHD film refers to the film of lubricating oil developed between a rolling element and a raceway of an operating rolling element bearing. The required technique had to provide accurate measurement, in an absolute sense, of the EHD film thickness so that minimum lubrication requirements could be established, as well as variations in other bearing dynamics as a function of film thickness changes. Attempts to use conventional film thickness measuring techniques resulted in data that was erratic, non-repeatable and generally unsatisfactory.
One conventional technique is to measure resistance across an operating rolling bearing to determine EHD film thickness. If a constant current is applied from the inner to the outer races for a rolling element bearing, the voltage across the bearing will change as a function of a resistance and the resistance will change as a function of EHD film thickness since the lubricating oil has a high dielectric constant. In an operating bearing a lubricant film is generated between the rolling elements and the raceways. The dielectric constant of the oil is high so even a thin film produces a very high resistance. However, the surfaces of the rolling elements and the raceways are not perfect so surface asperities periodically approach each other and when this happens the resistance becomes momentarily very low. This effect produces a very low signal to noise ratio when operating in the thin film regime, which is where the most critical data must be obtained.
The primary problem in using the resistance measurement across the operating bearing to establish the EHD film thickness is that the resistance path between the two races varies with time, due to the dynamics of the ball and the microscopic surface finishes of the ball and races. These resistance variations cause very high noise levels in the data and makes it very difficult to establish a mean resistance value that would relate to film thickness. Another problem with the resistance measuring technique is that, since the oil has a very low conductivity, as the film is established the resistance becomes very high and it doesn't change significantly with small changes in the film thickness. Since the regime of interest is where the film is fully developed, that is where there is no contact between the balls and races, the resistance measuring technique is not optimum.
The foregoing illustrates limitations known to exist in the present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.